This patent specification is in the field of magnetic resonance imaging (MRI) and more particularly in the field of 3D interleaved-cylindrical trajectory imaging.
An interleaved-cylindrical k-space trajectory can be envisioned as an arrangement of a number of individual helices (the xe2x80x9cinterleavesxe2x80x9d) as illustrated in FIGS. 1a and 1b, where FIG. 1a illustrates a top view of a starting position for the interleaves, with constant angular and radial steps, and FIG. 1b illustrates a side view of a single interleave with four turns. A helix such as seen in FIG. 1b can be generated in MRI by two oscillating and, with respect to each other, phase-shifted gradients in combination with a constant gradient in the third dimension. Increasing the amplitude of the oscillating gradients increases the radius of the traced helix. By varying the helical radius by a fixed increment and arranging the helices with identical radii along the circumference of the trajectory projection along the direction of the constant gradient, a set of concentric cylinders is formed, as seen in FIG. 1a. An approach of this type is discussed in U.S. Pat. No. 5,561,370, which is hereby incorporated by reference in this patent specification. The gradients can be along directions different from those illustrated.
This patent specification discloses a new approach that reduces blurring and distortions. While U.S. Pat. No. 5,561,370 illustrates in FIG. 8 oscillating gradients Gx and Gy that differ in both amplitude and in oscillating frequency for different interleaves, the new approach disclosed in this patent specification preferably maintains an essentially fixed oscillation frequency for all interleaves, independent of the amplitude of the oscillating gradient. As a result, the data points in a given slice perpendicular to the z-axis in Gif. 1b are acquired at essentially the same time relative to the exciting or refocusing radio frequency (RF) pulse(s). A consequence of this is that off-resonance signal sources appear as shifted along the z-direction in the image data set but are not as blurred or distorted as in the case of known MRI pulse sequences employing oscillating gradients.
FIGS. 1a and 1b illustrate a case in which the oscillating gradients conform to sinusoidal waveforms, which gives rise to circular projections. However, other waveforms can be used in accordance with this patent specification such as, without limitation, trapezoidal, triangular, and other waveforms.